The invention relates to a delay circuit comprising at least one all-pass network which has an input terminal for receiving an input signal, an output terminal for supplying an output signal, and a reference terminal for carrying a reference potential and which comprises an at least second-order filter section comprising three transconductors, which each have a first input, a second input, a first output and a second output, the first input of each of the three transconductors being coupled to the reference terminal, the second input of the first transconductor being coupled to the input terminal, the second input of the second transconductor being coupled to the second output of the first transconductor and to the first output of the third transconductor, the second input of the third transconductor being coupled to the second output of the second transconductor and to the output terminal, which filter section further comprises a first capacitor arranged between the second inputs of the first transconductor and the second transconductor, and a second capacitor arranged between the inputs of the first transconductor and the third transconductor.
Such a delay circuit may be employed for envelope-delay correction of filters, as a delay line whose delay may be variable or not, and in adaptive filters.
Such a circuit arrangement is known from the article "Integration of Analog Filters in a Bipolar Process" IEEE Jnl. of Solid State Circuits, Vol. SC-17, No. 4, August 1982, pp. 713-722. A transconductor is a voltage-controlled current source in which the proportionality factor between the output current and the input voltage is given by the transconductance. By means of a transconductor it is possible to simulate a resistor, while two coupled transconductors may be arranged to form a gyrator by means of which a capacitor and an inductor can be simulated. This means that using transconductors and capacitors it is possible to realize all filter circuits which can also be manufactured by means of conventional coils, capacitors and resistors. From the above article it is also known that all-pass network can be formed by means of transconductors and capacitors. FIG. 15 in the above article shows a first-order network and a second-order network comprising these elements. The transconductors have one input connected to ground and all the capacitors are floating. For the delay of signals of comparatively large bandwidth at least second-order all-pass networks are required in order to maintain deviations from the desired delay time comparatively small over the entire bandwidth. A drawback of such second-order all-pass networks is that parasitic effects, for example, as the result of stray capacitances, may show resonance effects. The gain of the all-pass network is then not equal to unity over the entire bandwidth of the signal and comparatively large frequency-dependent deviations from the desired delay occur.